This invention relates to the preparation of diaryl carbonates by oxidative carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply xe2x80x9ccarbonylationxe2x80x9d for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a heavy metal from Groups 8-10 of the Periodic table as printed, for example, in Handbook of Chemistry and Physics, 75th Edition (1994); i.e., a metal] from one of those groups which has an atomic number of at least 44, said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
A further development in the carbonylation reaction, including the use of compounds of other metals such as lead or cerium as cocatalysts, is disclosed in various patents including U.S. Pat. No. 5,498,789. Also required according to that patent is the use of quaternary ammonium or phosphonium halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package.
The commercial viability of the carbonylation reaction would be greatly increased if a less expensive compound could be substituted for the quaternary ammonium or phosphonium halide. It has been discovered, however, that substitution of such compounds as sodium bromide normally results in the isolation of the desired diaryl carbonate in low or insignificant yield.
The production of carbonates may be improved by including a metal-based cocatalyst along with the heavy metal catalyst from Groups 8-10. Suitable metal-based cocatalysts have been described broadly in U.S. Pat. Nos. 4,187,242 and 4,201,721 as compounds or complexes of copper, iron, manganese, cobalt, mercury, lead, cerium, vanadium, uranium, bismuth and chromium.
In U.S. Pat. Nos. 5,543,547 and 5,726,340, the use of carbonylation catalyst systems including palladium or an analogous metal, various cocatalytic metals which may include cerium, lead or cobalt, and an alkali metal or quaternary ammonium bromide is disclosed. Japanese Kokai 10/316,627 discloses a similar process in which the cocatalyst is a manganese or lead compound and a carboxylic acid amide or alkylurea is also present.
The use of a specific amide, N-methylpyrrolidone (hereinafter sometimes xe2x80x9cNMPxe2x80x9d), in a system employing a cobalt cocatalyst and, as an organic cocatalyst, a terpyridine or the like is disclosed in U.S. Pat. No. 5,760,272. The sole disclosed function of the NMP is to improve the selectivity to the formation of diaryl carbonate, as opposed to by-products such as biphenols. Further study of this system has revealed that it affords no improvement in diaryl carbonate yield defined in terms of xe2x80x9cturnover numberxe2x80x9d; i.e., the number of moles of diaryl carbonate formed per gram-atom of Group VIII catalytic metal present. This is contrary to the suggestion in the Japanese Kokai, which clearly teaches an improvement in yield.
It is of interest, therefore, to develop catalyst systems which include an inexpensive halide compound and which can efficiently produce diaryl carbonates.
The present invention provides a method for preparing diaryl carbonates which includes, a relatively inexpensive halide, a promoter compound which maximizes the effectiveness of said halide and a metal-containing cocatalyst. Also provided is a catalyst composition useful in such a method.
In one of its aspects, the invention provides a method for preparing a diaryl carbonate which comprises contacting at least one hydroxyaromatic compound with oxygen and carbon monoxide in the presence of an amount effective for carbonylation of at least one catalytic material comprising:
(A) a metal from Groups 8-10 of the Periodic Table having an atomic number of at least 44 or a compound thereof,
(B) at least one alkali metal halide or alkaline earth metal halide,
(C) at least one carboxylic acid amide, and
(D) at least one cocatalyst which is a compound of:
copper,
titanium in combination with zinc, copper or lead, or
cerium in combination with lead or manganese.
Another aspect of the invention is catalyst compositions comprising components A, B, C and D as described above, and any reaction products thereof.
Any hydroxyaromatic compound may be employed in the present invention. Monohydroxyaromatic compounds, such as phenol, the cresols, the xylenols and p-cumylphenol, are generally preferred with phenol being most preferred. The invention may, however, also be employed with dihydroxyaromatic compounds such as resorcinol, hydroquinone and 2,2-bis(4-hydroxyphenyl)propane or xe2x80x9cbisphenol Axe2x80x9d, whereupon the products are polycarbonate oligomers.
Other reagents in the method of this invention are oxygen and carbon monoxide, which react with the phenol to form the desired diaryl carbonate. They may be employed in high purity form or diluted with another gas such as nitrogen, argon, carbon dioxide or hydrogen which has no negative effect on the reaction.
For the sake of brevity, the constituents of the catalyst system are defined as xe2x80x9ccomponentsxe2x80x9d irrespective of whether a reaction between said constituents occurs before or during the carbonylation reaction. Thus, the catalyst system may include said components and any reaction products thereof.
Component A of the catalyst system is one of the heavy metals from Groups 8-10 of the Periodic Table, preferably palladium, or a compound thereof. Thus, useful palladium materials include elemental palladium-containing entities such as palladium black, palladium/carbon, palladium/alumina and palladium/silica; palladium compounds such as palladium chloride, palladium bromide, palladium iodide, palladium sulfate, palladium nitrate, palladium acetate and palladium 2,4-pentanedionate; and palladium-containing complexes involving such compounds as carbon monoxide, amines, nitrites, phosphines and olefins. Preferred in many instances are palladium(II) salts of organic acids, most often C2-6 aliphatic carboxylic acids, and palladium(II) salts of xcex2-diketones. Palladium(II) acetate and palladium(II) 2,4-pentanedionate are generally most preferred. Mixtures of the aforementioned palladium materials are also contemplated.
Component B is at least one alkali metal or alkaline earth metal halide, preferably a bromide such as lithium bromide, sodium bromide, potassium bromide, calcium bromide or magnesium bromide. Alkali metal bromides are especially preferred, with sodium bromide often being most preferred by reason of its particular suitability and relatively low cost.
Component C is at least one carboxylic acid amide, preferably a fully substituted amide; that is, one containing no NH groups including the amide nitrogen. It may be an aliphatic, aromatic or heterocyclic amide. Illustrative amides are dimethylformamide, dimethylacetamide (hereinafter sometimes xe2x80x9cDMAxe2x80x9d), dimethylbenzamide and NMP. Particularly preferred are NMP and DMA.
Component D is at least one cocatalyst which is a compound of copper, of a titanium-zinc, titanium-copper or titanium-lead mixture, or of a cerium-lead or cerium-manganese mixture.
Examples of lead compounds which may be employed are lead oxides such as PbO and Pb3O4; inorganic lead salts such as lead(II) nitrate; lead carboxylates such as lead(II) acetate and lead(II)propionate; lead alkoxides and aryloxides such as lead(II) methoxide and lead(II) phenoxide; and lead salts of xcex2-diketones such as lead(II) 2,4-pentanedionate. Mixtures of the aforementioned lead compounds may also be employed. The preferred lead compounds are lead(II) oxide, lead(II) aryloxides and lead(II) 2,4-pentanedionate.
Examples of cerium compounds are cerium carboxylates such as cerium(II) acetate, and cerium salts of xcex2-diketones such as cerium(III) 2,4-pentanedionate. Mixtures of the aforementioned cerium compounds may also be employed. The preferred cerium compounds are cerium 2,4-pentanedionates.
Examples of titanium compounds are inorganic titanium salts such as titanium(IV) bromide; titanium alkoxides and aryloxides such as titanium(IV) butoxide and titanium(IV) phenoxide; and titanium salts of xcex2-diketones such as titanium(IV) oxide bis(2,4-pentanedionate). Mixtures of the aforementioned titanium compounds may also be employed. The preferred titanium compounds are titanium(IV) alkoxides, aryloxides and 2,4-pentanedionates.
The preferred compounds of other metals are, for the most part, salts of xcex2-diketones and especially 2,4-pentanedionates.
In addition to the aforementioned reactants and catalyst system, a desiccant is sometimes present in the reaction system. The preferred desiccants are non-reactive materials such as molecular sieves, as illustrated by 3-xc3x85 ngstrom (hereinafter xe2x80x9c3Axe2x80x9d) molecular sieves. They are usually isolated from the other reactants, as by presence in a basket mounted to a stirrer shaft or the like. A frequently encountered feature of the present invention, however, is that desiccants are not necessary.
Component A is most often present in the amount of about 0.1-10,000 ppm by weight of the appropriate metal (usually palladium), based on the total of hydroxyaromatic compound and component C, and component B in the amount of about 1-2,000 mmol per equivalent of the metal of component A. Component D is generally present in the amount of about 1-200 gram-atoms of total metal per equivalent of the metal of component A.
The role of component C in the composition and method of the invention is believed to be to increase the degree of dissociation and ionization of the halide anion of component B, perhaps by forming a complex with the cationic portion of said component, although the invention is in no way dependent on this or any other theory of operation. The amount of component C employed will be an amount effective to optimize diaryl carbonate formation, in general by increasing the yield of the desired diaryl carbonate as evidenced, for example, by an increase in turnover number as defined hereinabove. This amount is most often about 1-60% by volume based on the total of hydroxyaromatic compound and component C.
The method of the invention is preferably conducted in a reactor in which the hydroxyaromatic compound and catalyst system are charged under pressure of carbon monoxide and oxygen and heated. The reaction pressure is most often within the range of about 1-500 and preferably about 1-150 atm. Gas is usually supplied in proportions of about 1-50 mole percent oxygen with the balance being carbon monoxide, and in any event, outside the explosion range for safety reasons. The gases may be introduced separately or as a mixture. Reaction temperatures in the range of about 60-150xc2x0 C. are typical. In order for the reaction to be as rapid as possible, it is preferred to substantially maintain the total gas pressure and partial pressure of carbon monoxide and oxygen, as described, for example, in U.S. Pat. No. 5,399,734, until conversion of the hydroxyaromatic compound is complete.
The diaryl carbonates produced by the method of the invention may be isolated by conventional techniques. It is often preferred to form and thermally crack an adduct of the diaryl carbonate with the hydroxyaromatic compound, as described in U.S. Pat. Nos. 5,239,106 and 5,312,955.